1. Field of the Invention
The invention relates to conjugated polymers and methods of making the same.
2. Related Art
Organic π-conjugated polymers are attractive materials for use in the active layer, as they combine good absorption and emission characteristics with efficient charge carrier mobility and have the ability to be solution processed onto flexible substrates. Recent advances in the field have seen organic field effect transistors (OFET) achieve charge carrier mobility on the order of 1.0 cm2V−1 s−1 [1] and organic photovoltaic (OPV) devices reach power conversion efficiencies over 7% [2]. While these results are promising for the field, there still exits a complexity of correlating molecular structure to optical and electronic properties. Among the available narrow band-gap materials, donor-acceptor copolymers based on cyclopenta[2,1-b:3,4-b′]dithiophene (CDT) and benzothiadiazole (BT) have attracted considerable attention due to the high charge carrier mobility and excellent photovoltaic performance. Müllen and co-workers have eloquently demonstrated that CDT-BT copolymers with linear side chains and high molecular weights, p-type FETs with mobilities on the order of 1.4-3.3 cm2 [3].
The incorporation of a nitrogen atom into the acceptor unit of CDT-BT copolymers results in the narrowing of the optical bandgap and the emergence of these materials to selectively bind Lewis acids [4]. The replacement of the BT unit with the pyridal[2,1,3]thiadiazole (PT) acceptor unit results in a higher electron affinity across the polymeric backbone leading to a decreased LUMO level of polymer. Copolymers based on PT and carbazole reported by Lerclerc et al. [5] have fairly low molecular weights (ca. 4-5 kDa), and the efficiencies of the fabricated solar cells (under 1%) are much lower than predicated. You and co-workers have demonstrated that by introducing two alkyl chains to the 4-position of the thienyl unit could lead to a more soluble PT based acceptor (DTPyT), and allows access to polymers with high molecular weights and excellent photovoltaic efficiency up to 6.32% [6]. In each case, however, the nature of the step-growth polymerization strategy leads to these polymer systems having a regiorandom origination of the pyridal-N atom along the polymeric backbone.